Irradiation of homopolymers of isobutylene or copolymers of isobutylene and isoprene by prior known techniques has led to degradation. This has precluded the use of these compounds in environments where they would be subjected to irradiation, despite the advantages inherent in such products. It has also precluded their use in compositions and blends such as are often used in the rubber industry when it is intended that such compositions or blends should be cured by irradiation. This is especially important in tire compounding. Tires and their various components often contain a number of rubbers, both natural and synthetic. It has been proposed that tires could be commercially cured or vulcanized by high energy ionizing radiation. However, because so many tire blends contain these ingredients, especially as butyl rubber, the art has not adopted the new techniques.
It is already known to degrade polymers and copolymers of isobutylene by radiation techniques. U.S. Pat. No. 3,080,306 describes the preparation of viscosity improving materials for use as lubricating oil additives by a process in which the molecular weight of the polymeric materials is adjusted downwardly by exposing the polymeric compounds to ionizing radiation after mixing them with certain halogenated hydrocarbons.
Isobutylene is available in several commercial grades ranging in molecular weight from about 225,000 to 6,000,000. One of its commercial trade designations is Vistanex. It is also available as a copolymer with from 0.6 to 3 mole percent of isoprene. The copolymer is known as butyl rubber. Its molecular weight range is also about 225,000 to 6,000,000. It is available in many commercial forms and grades. The materials are useful as replacements for natural rubber, in the formation of adhesive compounds such as pressure sensitive tapes and for a variety of other purposes. Important uses of butyl rubber include the formation of inner tubes, and in other products which must be substantially impervious to a gas such as air ... for example gas masks. It has also been compounded for general tire purposes, especially for tires for farm machinery and other off the road vehicles which are operated at low speeds and may be idle for long periods. Even in these uses, however, the rubber is not totally satisfactory because of its tendency to deteriorate.
Butyl rubber has certain properties that make it costly to process. The unvulcanized rubber is subject to cold flow and is very sticky, making it difficult to handle and store. Rates of vulcanization by the usual procedures are much lower than for natural or SBR rubber. Higher temperatures and longer times are generally required for vulcanization. Its extrusion rate is relatively slow.
Polyisobutylene and butyl rubber, because of relatively high resistance to the passage of air, are often used as gasket material. But they are not so used in areas where the gasket would be exposed to radiation because they are subject to deterioration by radiation.
Because of the importance of the problem, and particularly because of the size of the markets involved the art has spent much time and effort attempting to improve polyisobutylene and butyl rubber to make them processable by radiation techniques.